Can I protest?

I wonder if anyone can point me in the right direction with all this. It was once mentioned that visa holders not being able attend demonstrations and protests although I have not been able to find any direct information about this, and if it is actually in law at the moment.

And if something like this was/is in place does it include attending talks, conferences and what is the definition of a protest? If one attends a gathering which is anti-"whichever" political party does that make it an anti-government protest?

Example, with the demonstrations taking place at the moment which involve talks and protest about the Pope visiting Britain that is different than say an anti-war protest which directly has to do with the government? Same with anti-racism rallies ... isn't that a total different kind of demonstration?

Whoever said that about visa holders didn't know what they were talking about.

You have the right to protest...You don't have the right to break the law. 

I've also never heard that you can't protest.

However, it's worth keeping in mind that UK police techniques such as kettling (seen in last year's G20 protests) mean that it's possible to go to a protest, not do anything wrong, and still end up detained, details and photo taken, etc. I have no idea whether this would affect any future 'good character' test you might have to pass for citizenship or whatever. So it might be good to stick to demos that are unlikely to draw a big police presence or be controversial or what have you.